PROJECT SUMMARY There are 3.2 million persons in the United States chronically infected with hepatitis C virus (HCV) with a 1- 2.4% prevalence during pregnancy. The recent October 2014 approval of the fixed dose combination, containing the NS5B polymerase inhibitor sofosbuvir (SOF) 90 mg and the NS5A inhibitor ledipasvir (LDV) 400mg, marked a new era of IFN and ribavirin free, directly acting antiviral treatment for HCV. A 12 week treatment course of LDV/SOF resulted in a 99% cure rate when given as a once-a-day oral pill. Based on the animal model data submitted to the FDA, this drug combination was given a pregnancy category B designation, even though there is currently no experience with LDV/SOF in pregnant women. Pregnancy is a time when women are uniquely motivated to engage in activities which are geared toward improvement of their own health and ensuring the health of their unborn child. As such, pregnant women have frequent prenatal care visits; and health care interventions, such as antiviral therapy and monitoring, can be easily integrated into the existing healthcare infrastructure of prenatal care. The benefits of HCV treatment are numerous, including prevention of severe liver disease, hepatocellular carcinoma, and liver transplantation, as well as improvements physical, emotional and social health. The most recent guidelines by the Infectious Disease Society of America recommend that all HCV-infected persons receive treatment. The antenatal period represents an ideal window of opportunity for treatment of HCV in pregnancy due to increased antenatal health care utilization and prevention of perinatal transmission of HCV to the infant. Safe administration of drugs in pregnancy may require dose adjustment due to the pregnancy-induced physiologic alternations. Therefore, careful pharmacokinetic (PK) evaluation is a critical first step to ensure safe administration of drugs to both the mother and the developing fetus. In this R21 application, we propose a single-arm, single-center, open label Phase 1 evaluation of the PK and safety of treating HCV with a 12 week course of LDV/SOF in 15 HCV-infected pregnant women. Therapy will be initiated at approximately 24 weeks of gestation. In this study we will determine: 1) if the PK of the LDV and SOF are similar in pregnancy as compared to those in nonpregnant women, 2) if the viral response to LDV/SOF treatment in pregnancy is similar to that observed in nonpregnant women, and 3) if there are any initial maternal or neonatal safety concerns detected with antenatal LDV/SOF administration compared with HCV-infected historical controls delivered at our institution. From the findings of this study, future studies will seek to optimize the dose, gestational age timing and treatment duration of LDV/SOF during pregnancy. Antenatal HCV treatment will improve maternal health, prevent further HCV transmission in the community and perinatal HCV transmission to the child, and thus enhance the long-term health of two generations.